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Liu, Lijun (China University of Petroleum) | Huang, Zhaoqin (China University of Petroleum) | Yao, Jun (China University of Petroleum) | Lei, Qinghua (ETH Zurich) | Di, Yuan (Peking University) | Wu, Yu-Shu (Colorado School of Mines) | Liu, Yongzan (Texas A&M University)
The object of this work is to develop an efficient coupled hydro-mechanical numerical model for two-phase flow in fractured vuggy porous media. The fluid flow in matrix and fractures is described by two-phase Darcy's equation, and the free flow in vugs is simplified with the assumption of multiphase instantaneous gravity differentiation. The modified Barton-Bandis's constitutive model is used to handle the nonlinear deformation of fractures. The fluid pressure is applied on the vug boundaries to model the vug deformation. Then the finite-volume (FVM) and finite-element (FEM) methods are used for space discretization of the flow and geomechanics equations, respectively. The coupled problem is iteratively solved using fixed-stress splitting method. Then a set of 2D and 3D simulation cases are conducted to investigate the effects of fractures and vugs on the flow and geomechanical behaviors. The results show that vugs can hinder the water breakthrough due to their storage effect, while water can quickly break through in the high-conductivity fractures. The significant effect of gravity on the saturation distribution can be observed in the 3D case. Besides, the stress concentration is much more obvious when vugs are present. 1. INTRODUCTION The coupled hydrology and mechanics of fractured vuggy porous media is an important issue in several fields, such as oil recovery in fractured vuggy carbonate reservoirs and mining engineering. The fractured vuggy porous media is usually characterized by its multiscale pore space, including porous matrix, natural fractures, and vugs (Okabe and Blunt, 2007). Due to the co-existence of porous flow, fracture flow and free flow, as well as their coupling with the deformation, the hydro-mechanical modeling in fractured vuggy porous media remains challenging, especially for two-phase flow. A series of models have been developed to study fluid flow in fractured vuggy porous media, including equivalent continuum model (Popov et al., 2009; Huang et al., 2011), triple continuum model (Wu et al., 2011), and discrete fracture-vug model (Girault & Rivière, 2009; Yao et al., 2010; Liu et al., 2020a). However, the former two models are too simplified to capture the dominating flow of large-scale fractures and vugs (Zhang et al., 2016), and most of current discrete fracture-vug models focus on single-phase flow.
Yao, Jun (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Huang, Zhaoqin (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Li, Yajun (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Wang, Chenchen (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Lv, Xinrui (School of Petroleum Engineering, China University of Petroleum, P.R. China)
Abstract Modeling and numerical simulations of fractured vuggy porous media is a challenging problem due to the presence of cavities called macro vugs which are connected via discrete fractures networks. The main difficulty is the co-existence of porous and free-flow region in such media on macro scale. In this study, a novel conceptual discontinuum model i.e. discrete fracture-vug network (DFVN) model has been proposed to this problem. In DFVN conceptual model, naturally fractured vuggy porous rock masses are considered as a composite porous material, consisting of (1) macro fractures system, (2) porous rock matrix system, and (3) macro vugs system. Macro fractures and vugs are embedded in porous rock, and the isolated vugs are connected via discrete fracture network. We model the fractured vuggy media on macroscopic scale using Navier-Stokes equations within the vugular region, Darcy's law within the porous flow region including porous rock matrix system and macro fractures system, and a Beavers-Joseph-Saffman boundary on the interface between two regions. A standard Galerkin finite element method is implemetated for the solution of DFVN model. A good match with analytical and numerical solutions for Poiseuille flow in a free/porous channel was achieved, which verified the accuracy of our finite element numerical scheme. Both 2D DFVN models with homogeneous isotropic rock matrix and heterogeneous anisotropic rock matrix are simulated and studied. The numerical results have shown that DFVN model provides a natural way of modeling realistic fluid flow in fractured vuggy porous media.
Kang, Zhijiang (SINOPEC Expl & Prod Rsch Inst) | Wu, Yu-Shu (Lawrence Berkeley Laboratory) | Li, Jianglong (Research Inst. of Petroleum Exploration and Development, SINOPEC) | Wu, Yongchao (Research Inst. of Petroleum Exploration and Development, SINOPEC) | Zhang, Jie (Research Inst. of Petroleum Exploration and Development, SINOPEC) | Wang, Guangfu (Research Inst. of Petroleum Exploration and Development, SINOPEC)
Abstract A multicontinuum conceptual model is presented and implemented into a three-dimensional, three-phase reservoir simulator, using a generalized multicontinuum modeling approach. The conceptual model, proposed for investigating multiphase flow and displacement through naturally fractured vuggy carbonate reservoirs, is based on observation and analysis of geological data, as well as on core examples from the carbonate Tahe Oil Field in China. In this conceptual model, naturally fractured vuggy rock is considered to be a triple-continuum medium, consisting ofhighly permeable and well-connected large-scale fractures; low or impermeable rock matrix; and various-sized vugs or cavities. The base matrix system may contain many small or isolated cavities (of centimeters or millimeters in diameter), and large cavities (or vugs) ranging from centimeters to meters in diameter. Vugs may bedirectly connected to large fractures, indirectly connected to large fractures through small fractures or microfractures, or isolated from large fractures by rock matrix. Similar to the conventional double-porosity concept, the fracture continuum is responsible for the occurrence of global flow, whereas vuggy and matrix continua (mainly providing large-storage space of source/sink) are locally connected to each other as well as interacting with globally connected fractures. In the numerical implementation, a control-volume, integral finite-difference method is used for spatial discretization, and the resulting discrete nonlinear equations for the three-phase fluids, coupled with each continuum, are solved fully implicitly by Newton iteration. The numerical scheme, verified by comparing its results against those of available analytical solutions, is used to simulate water-oil flow through the fractured vuggy reservoirs of Tahe. Introduction Naturally fractured reservoirs existing throughout the world represent a significant amount of the world oil and gas reserves. Since the 1960s, studies of flow and transport in fractured rock have received increasing attention, and significant progress has been made in numerical modeling of flow and transport processes in fractured reservoirs. Research efforts, driven by the increasing need to develop petroleum and geothermal reservoirs (as well as to resolve subsurface contamination problems), have developed many numerical modeling approaches and techniques. In the past decade, the petroleum industry has faced a growing demand for oil and natural gas, while at the same time few new oil reserves have been found worldwide. The efficient development of naturally fractured reservoirs has become a top priority of the oil industry. Because of the known low oil recovery rates (in general) from naturally fractured reservoirs, interest in enhancing oil and gas recovery from such reservoirs has grown, with more investigations conducted for multiphase flow and transport phenomena in fractured reservoirs. Since the 1970s, in parallel to the development in the oil industry, environmental concerns over subsurface contamination have motivated many studies of fluid, chemical, and heat transport in variably saturated fractured formations. Moreover, suitability evaluations for underground geological storage of high-level radioactive waste in fractured rock have generated renewed interest in investigations of multiphase and radionuclide transport in a fractured geological system. Even though significant progress has been made towards the understanding and modeling of flow and transport processes in fractured rock since the 1960s, most of those studies have focused on naturally fractured reservoirs, without including cavities. Recently, driven by the need to develop underground natural resources, and by environmental concerns, characterizing vuggy fractured rock has currently received attention, because many naturally fractured vuggy reservoirs have been found worldwide and can significantly contribute to reserves of oil and gas. Significant interest is being generated in investigating vuggy fractured reservoirs.
The 500 million barrel Midale oilfield is part of a trend of large Mississippian oil accumulations located in southeastern Saskatchewan, along the northern margin of the Williston Basin. The field was discovered in 1953 and developed on 80-acre spacing. In 1962 Midale was unitized for waterflooding, with 83 320-acre inverted ninespot patterns (Figure 1).
Waterflood performance in Midale is dominated by a system of oriented natural vertical fractures typically spaced 1-4 feet apart. Production wells located "ontrend" from injectors (aligned with the fractures) showed sharp early response to waterflood. In contrast, response of "off trend" producers was smooth and delayed. Oil produced to-date represents recovery of 20% of the original oil in place (OOIP); ultimate waterflood recovery is predicted to be only 24% OOIP. Current watercut in the mature operation is about 80% (Figure 2). Even with the natural fractures, the Midale Unit is a low-productivity reservoir, with average production rates of 75-100 STB/D/well.
A major reservoir and process mechanism characterization effort began in the mid-1980s to guide ongoing Midale field development opportunities. A detailed study team consisting of a geologist, a petrophysicist, and production, reservoir, and research engineers spent three years analyzing waterflood performance. The large-scale effort was dictated by the complexity of predicting performance in a heavily fractured reservoir. Obviously, good communication and cross-discipline integration of expertise was a key to the project's success. The resulting reservoir and process model honors all available data and can match performance of the field-scale waterflood as well as a tertiary CO2 flood pilot.